\subsection{XMPP}
\pages{3-4}

The \term{Extensible Messaging and Presence Protocol (XMPP)} is a distributed,
XML-based protocol for real-time communication. Its core functionalities are
specified in RFCs~6120~\cite{rfc6120} and RFC~6122~\cite{rfc6121}, while protocol
extensions are usually defined by the XMPP community in \term{XMPP Extension
Proposals (XEPs)}.

\subsubsection{Addressing}

Every user account in XMPP is addressed by a globally unique identifier, called
the \term{Jabber ID (JID)}~\cite{rfc6122}. It has the form
\code{localpart@domainpart/resource}, where \code{domainpart} is the DNS name of
an XMPP server, and \code{localpart} is the name of a user account on that
server. Since a user can be logged in from multiple clients, the \code{resource}
part is a string chosen by the user to distinguish those clients. Only the part
\code{localpart@domainpart} (the \term{bare JID}) is needed to identify a user,
the resource is only needed for routing between client and server.

\subsubsection{Architecture}
\begin{wrapfigure}{r}{0.4\textwidth}
%\begin{figure}[htop]
  \centering
  \includegraphics[width=0.4\textwidth]{fig-xmpp-architecture-mock.jpg}
  \caption{XMPP architecture}
  \label{fig:xmpparch}
%\end{figure}
\end{wrapfigure}

The original architecture underlying XMPP strongly leans on the established
design of Internet Mail, and an example is depicted in Fig.~\ref{fig:xmpparch}.
The distributed network is formed by \term{XMPP servers} on one hand, which make
up the always-on backbone of the network used for routing message, and manage
user accounts and statuses. On the other hand, \term{XMPP clients} represent a
single logged-in user and are the interface for communication with other users.

Every client communicates only with the server that manages the respective user
account which is configured in the client, as given in the user's JID. The
server then routes the messages to their recipients, using the JID to determine
the correct server for a message to be sent to. Finally, the receiving server
sends the message to a client where the receiving JID is logged in. If the user
is not logged in at the time the message is sent, the server can store it for
the user and deliver it on the next login.

XMPP strongly relies on DNS Service Discovery (see Section~\ref{sec:dnssd}) to
determine the server being in charge of a domain. For example, the server who
manages the users for the domain \code{example.org} is given by the SRV record
\code{\_xmpp-server.\_tcp.example.org}.

\subsubsection{Communication primitives}

All communication over XMPP is based on XML. To minimize communication overhead,
only fragments of XML, called \term{stanzas}, are sent between hosts. A stanza
is always well-formed as a whole; it consist of a root element, which also
includes routing attributes (\code{to} and \code{from}), and its optional child
elements.

On top of that, living connections between hosts are represented by \term{XML
streams}. The client initiates a connection by sending an XML declaration
followed by an opening \code{<stream>} tag. The server then responds also with
an opening \code{<stream>} tag. The client then performs SASL authentication and
binds its stream to a resource for proper addressing. If this process succeeded,
both client and server can send an unlimited number of stanzas, until the
connection is closed by one side by sending an closing \code{</stream>} tag. The
other side then has the chance to send all outstanding stanzas and likewise
closes its stream. If both streams are closed, the underlying TCP connection is
terminated.

\todo[Example stream]

\subsubsection{Publish/Subscribe and Presence}

Typically, a user wants to chat with a more or less fixed set of other users,
whose JIDs she needs to know, so she needs some kind of ``address book'' that
remembers the JIDs for her. In XMPP, this is address book is called
\term{roster}, and it also shows the users' willingness to chat (``presence'').
In order to see their chat status (which can be  one of ``online'', ``offline'',
and several ``away'' or ``do not disturb'' states), a user needs to subscribe to
the other user's status. The mechanism behind this is called
\term{Publish-Subscribe} and is specified in XEP-0060~\cite{xep-0060}. It can
be used to notify interested users about changes in personal information, and
implements the classic Observer pattern.

A user publishes information by creating a \term{node} on the XMPP server, which
acts as a handle for the data. Interested users can then query the server for
nodes, and request subscription to them. When the owner of the node confirms the
subscription request, subscribers get notified whenever the owner updates the
respective node.

All communication takes place between the client and the server over \code{<iq>}
(``information query'') stanzas.

\subsubsection{Multi-User Chats}

Besides one-to-one messaging, XMPP also allows users to create multi-user
chat rooms, which is specified in \cite{xep-0045}. Each chat room is given a
unique JID to which the users send their messages to. Each incoming message is
then dispatched to all users which have joined the room.

To join a room, the user sends a \code{<presence>} stanza to the room JID, where
the resource part of the room JID specifies the desired nick name.

\subsubsection{XMPP Serverless Messaging}\label{sec:xsm}
\pages{1}

To overcome the need for a central server and authentication, XMPP Serverless
Messaging~\cite{xep-0174} allows XMPP clients on a network to build a
peer-to-peer mesh network and chat directly with each other. This feature was
first introduced by Apple as part of their \term{Bonjour} project, and nowadays
it is also available in many other XMPP clients.

With XMPP Serverless Messaging, XMPP clients simply open a port on the host, and
then rely on mDNS and DNS-SD (see Section~\ref{sec:dns})
to publish instance names in the domain \code{\_presence.\_tcp.local}. For
example, if Juliet uses her machine (named \code{capulet}) with serverless
messaging, her client would publish the following four mDNS records:

\begin{itemize}
  \item an A record \code{capulet.local}, specifying her IP address,
  \item an SRV record \code{juliet@capulet.\_presence.\_tcp.local}, specifying
    the port on which her XMPP client listens, and refering to
    \code{capulet.local} as the host name
  \item a PTR record \code{\_presence.\_tcp.local} for service discovery,
    pointing to \code{juliet@capulet.\_presence.\_tcp.local}
  \item and a TXT record \code{juliet@capulet.\_presence.\_tcp.local} specifying
    more information about her (e.~g. her online status, contact data, etc.)
\end{itemize}

When other clients in the same network enumerate the available services by
querying \code{\_presence.\_tcp.local}, they notice Juliet's presence and add
her to the roster automatically. In that way, XMPP users can see who is
currently available for communication. To start a chat session, clients initiate
a TCP connection over the advertised ports, open their XML streams, and send
message or IQ stanzas like they would to an XMPP server. Presence is managed
over the corresponding TXT record in the mDNS. To go offline, a client
announces the deletion of its mDNS records.

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